A known method of manufacturing permanently structured magnetic tape is described in U.S. Pat. No. 4,023,204, and is shown schematically in FIG. 1. A flexible substrate such as a polyester film (1) is coated with a liquid slurry having anisotropic magnetic particles (such as for example acicular ferric oxide particles) (2) which have a generally random orientation. The coated substrate is moved in the direction of the arrow (3) past a first permanent magnet (4), which makes an oblique angle with the direction of motion of the substrate. This causes the particles (2) to become aligned as shown in area 5. The coated substrate continues to move in the direction of the arrow (3) past a further magnet (6), which is an electromagnet, and thus can generate a controllably variable magnetic field. This magnet is arranged to make an oblique angle with the direction of the first magnet. By switching the strength of the magnetic field being generated by the further magnet, the particles (2) become aligned as shown in area 7. The slurry is then solidified to fix the particles in place in the orientation shown in area 7. With this arrangement it will be noted that the magnetic properties of the coating are constant across the width of the substrate (8), but change in the direction of otion of the substrate (3). The substrate 1 is subsequently slit along its length (i.e. direction of arrow 3) to provide thin lengths of magnetic tape.
In a modification of the above method, disclosed in U.S. Pat. No. 3,873,975, data can be coded in direction 8 across the tape by substituting a segmented magnet with a plurality of independently controllable write heads for the electromagnet 6. This can provide the advantage that after slitting the substrate, each length of tape can have different data. This method is reasonably satisfactory for making permanently structured magnetic tape for use on IS0781X cards, where the width of tape reels required lie between 11 and 15 mm. Typically, 15 independent channels are created simultaneously across the substrate, each one 11-15 mm wide.
However, the method is unsuitable for manufacturing narrow tapes of the order of 1 or 2 mm across as might be embedded into banknotes, because of the lateral spreading of the magnetic field from each head of the further magnet. The method also has the disadvantage that many independent channel driver circuits are required, one for each write head. To make 1 mm wide threads, the required number of channels will increase by an order of magnitude (for example from 15 to 150). The cost of providing so many circuits, and checking the calibration of each one periodically, becomes increasingly high for narrow threads. It is an object of the present invention to mitigate the above disadvantages.